Purpose: In experiments where the outcome of movements result in explicit monetary rewards and penalties, subjects typically plan movements that come close to maximizing their expected gain. But what if an economically optimal movement proves to be intrinsically stressful to the organism? Would subjects trade gain to avoid pain? And if they did so, how would they price biomechanical discomfort in monetary terms? We tested how degree of discomfort affected movement planning in a simple grasping task.

Methods: Subjects attempted to rapidly grasp circular disks (50 mm diameter, 10 mm high). The edge of each disk was marked with two reward regions symmetrically-placed on the circumference. If the thumb and forefinger contact points both fell within the reward regions the subject received a monetary reward and otherwise a penalty. A grasp aimed at the centers of the reward regions would maximize expected reward but such a grasp varied in comfort with rotation angle. From trial to trial we rotated the reward regions, forcing the subject to trade off comfort and expected gain. In one condition (“narrow”) the reward regions spanned 40 degrees, in a second 60 degrees (“wide”). Deviating from the center was potentially more costly to the subject in the narrow than in the wide condition.

Results: Participants systematically traded a portion of their potential gain to achieve a more comfortable grasp position. The relationship can be described by a monotonic function of wrist rotation angle. This interrelation implies that biomechanical constraints may have a direct influence on the estimated usefulness of a movement. Our findings demonstrate that the motor system includes biomechanical comfort as one factor component of planning movements that maximize expected gain.